1. Field of the Invention
This invention relates generally to radio frequency resonators and, more particularly, to inner conductors of radio frequency coaxial resonators.
2. Description of the Related Art
Coaxial resonators are used in a wide variety of applications, including filters and oscillators used in communication systems. Coaxial resonators offer advantages over other resonator construction techniques, such as discrete components, microstrip, and transmission line filters that can suffer from high dissipation, resulting in lower Q-values. In addition, these techniques can require large physical dimensions for proper operation.
Coaxial resonators can provide improved Q-values over other resonator construction techniques. FIG. 1 is a side elevation of a typical coaxial resonator 100 of conventional construction. The FIG. 1 resonator includes an inner conductor 102 placed within a cavity 104 that is formed from an enclosure having sidewalls 106, a bottom wall 108, and a top wall 110. The interior surface 111 of the enclosure cavity 104 is conductive. The inner conductor 102 is attached to the enclosure at the bottom wall 108, thereby providing an electric short-circuit path between the cavity enclosure 104 and the inner conductor 102. The free end 112 of the inner conductor 102 is an open-circuit, providing capacitive coupling between the inner conductor and the inner surface 111 of the cavity enclosure.
Coaxial resonators constructed as illustrated in FIG. 1 can provide the benefit of relatively high Q-values. The length of the inner conductor 102 for these types of coaxial resonators is on the order of one fourth of the wavelength (λ/4) of the desired operating frequency. The length of the inner conductor that is required for such quarter-wavelength conductors can be a drawback when trying to minimize the size of the resonator.
FIG. 2 illustrates a resonator 200 that maintains the advantage of a high Q-value while decreasing the length of the inner conductor for a given operating frequency. In FIG. 1 and FIG. 2, and in all the drawings, like reference numerals refer to like structures. As illustrated in FIG. 2, a transverse disk 202 is added to the free end 112 of the inner conductor 102. The disk 202 has a larger diameter than that of the inner conductor 102. An advantage of the resonator 200 illustrated in FIG. 2 is that the surface area of the disk 202 and the distances between the disk 202 and the interior wall surfaces 106, 108, 110 of the cavity enclosure can be dimensioned to increase the capacitance between the free end of the inner conductor and the cavity 104. Increasing the capacitance between the free end of the inner conductor and the cavity allows the overall length of the inner conductor to be decreased for a given operating frequency. Thus, a resonator of more compact dimensions can be provided.
A drawback to the resonator illustrated in FIG. 2 is that additional manufacturing steps are required as compared with the FIG. 1 construction to make the disk 202 and to attach it to the free end 112 of the inner conductor 102. A technique to overcome this drawback is to machine the inner conductor 102 and disk 202 from a single piece of raw material, starting with a solid block. While this technique overcomes the problems of making a separate disk and attaching the disk to the free end of the inner conductor, the machining process is relatively expensive and time consuming.
Another technique to overcome the additional manufacturing steps required to make the disk and attach it to the free end of the inner conductor is to manufacture the inner conductor using a deep-drawing method. In a deep-drawing method, a piece of raw material, typically a sheet of material, is held around its edges and is struck repeatedly in its center by a tip of an impact tool. As the tool strikes the material, the material is drawn in the direction of the impact, thereby forming a projection that extends from the raw material in the direction of impact. After the projection has reached a desired length, the projection is cut from the material. The projection can be cut from the sheet material so that a portion of the sheet material remains with the projection to form a transverse edge. In this way, an inner conductor with free-end disk can be formed. Although the projection cutting process can form the end of the projection to a desired shape, the repeated striking and the cutting processes are generally expensive and time consuming.
There is therefore a need in the art for an improved apparatus and method of making flanged conductive bodies for use as inner conductors in resonators.